From d080051e3240bec2c33157032407137105aec039 Mon Sep 17 00:00:00 2001
From: Sarath Francis <sarathfrancis90@gmail.com>
Date: Sat, 18 Jul 2026 03:27:33 -0400
Subject: [PATCH] fix: fall back to default checkbox renderer when extension
 returns false

A renderer extension named after a built-in token can return false to
fall back to the default renderer. The Parser decides that from a list
of built-in token type names, and 'checkbox' was never added to either
the block or the inline list when the checkbox token was introduced.

The result is that a `checkbox` renderer extension returning false
emits nothing instead of falling back, so the checkbox silently
disappears from the task item.
---
 src/Parser.ts            |  4 ++--
 test/unit/marked.test.js | 26 ++++++++++++++++++++++++++
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/src/Parser.ts b/src/Parser.ts
index 636a8287b9..1fa984a2ad 100644
--- a/src/Parser.ts
+++ b/src/Parser.ts
@@ -50,7 +50,7 @@ export class _Parser<ParserOutput = string, RendererOutput = string> {
       if (this.options.extensions?.renderers?.[anyToken.type]) {
         const genericToken = anyToken as Tokens.Generic;
         const ret = this.options.extensions.renderers[genericToken.type].call({ parser: this }, genericToken);
-        if (ret !== false || !['space', 'hr', 'heading', 'code', 'table', 'blockquote', 'list', 'html', 'def', 'paragraph', 'text'].includes(genericToken.type)) {
+        if (ret !== false || !['space', 'hr', 'heading', 'code', 'table', 'blockquote', 'list', 'checkbox', 'html', 'def', 'paragraph', 'text'].includes(genericToken.type)) {
           out += ret || '';
           continue;
         }
@@ -136,7 +136,7 @@ export class _Parser<ParserOutput = string, RendererOutput = string> {
       // Run any renderer extensions
       if (this.options.extensions?.renderers?.[anyToken.type]) {
         const ret = this.options.extensions.renderers[anyToken.type].call({ parser: this }, anyToken);
-        if (ret !== false || !['escape', 'html', 'link', 'image', 'strong', 'em', 'codespan', 'br', 'del', 'text'].includes(anyToken.type)) {
+        if (ret !== false || !['escape', 'html', 'link', 'image', 'checkbox', 'strong', 'em', 'codespan', 'br', 'del', 'text'].includes(anyToken.type)) {
           out += ret || '';
           continue;
         }
diff --git a/test/unit/marked.test.js b/test/unit/marked.test.js
index b604c0103e..07e58f8578 100644
--- a/test/unit/marked.test.js
+++ b/test/unit/marked.test.js
@@ -396,6 +396,32 @@ describe('marked unit', () => {
       assert.strictEqual(html, '<h1>extension1 RENDERER EXTENSION</h1>\n<pre><code>extension2 TOKENIZER EXTENSION\n</code></pre>\n');
     });
 
+    it('should fall back to the default checkbox renderer if the extension returns false', () => {
+      marked.use({
+        extensions: [{
+          name: 'checkbox',
+          renderer(token) {
+            return token.checked ? false : '<span class="todo"></span> ';
+          },
+        }],
+      });
+      const html = marked.parse('- [x] one\n- [ ] two\n');
+      assert.strictEqual(html, '<ul>\n<li><input checked="" disabled="" type="checkbox"> one</li>\n<li><span class="todo"></span> two</li>\n</ul>\n');
+    });
+
+    it('should fall back to the default checkbox renderer in a loose list if the extension returns false', () => {
+      marked.use({
+        extensions: [{
+          name: 'checkbox',
+          renderer() {
+            return false;
+          },
+        }],
+      });
+      const html = marked.parse('- [x] one\n\n- [ ] two\n');
+      assert.strictEqual(html, '<ul>\n<li><p><input checked="" disabled="" type="checkbox"> one</p>\n</li>\n<li><p><input disabled="" type="checkbox"> two</p>\n</li>\n</ul>\n');
+    });
+
     it('should walk only specified child tokens', () => {
       const walkableDescription = {
         extensions: [{